Wireless device impairments can be cancelled by post processing a received signal when deteriorating device parameters are sufficiently well known. Theoretically the parameters can be determined during production test on a sample-by-sample basis and stored within the device or elsewhere in the overall wireless system. Later, during device operation, this information can be used by an algorithm to correct the received, impaired signal. To determine Local Oscillator (LO) and receiver internal impairments, a common method is to inject a test signal into the signal receiving port during a production test and evaluate the obtained receiver output signals in a post processing step. However, this production test approach assumes stable impairments. Also, fusing and production calibration imposes additional effort and cost.
More suitable is an auto calibration procedure to be applied at every device start-up or during operation right before the receiver begins normal operation. The generation of a precise test signal supplied from the production tester is no longer available. This approach requires an on-board signal source, but such a source is supposed to be precise, which typically increases device cost or area significantly.
A quadrature modulator architecture's own transmitter output signal can be used as test signal because of the signal's composite modulation in contrast with the non-modulated LO signal. However, in polar transmitter architectures, the on-board approach is not possible because the same phase modulated LO is also used as LO for the feedback receiver. As a result, the received and down converted feedback signal has a constant relative phase and the demodulated signal maps only to a pure radial trajectory in the constellation diagram rather than a circular distributed trajectory. Maintaining the calibration approach but introducing a precise on-board signal source comparable to the one from the production tester is too costly because of the required fidelity and precise frequency control across a greater frequency range.